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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var a = 18;
        f1();
        function f1() {
            var b = 9;
            console.log(a);
            console.log(b);
            var a = '123';
        }

        //相当于执行下面的代码
        /*
        var a;
        function f1(){
             var b;
             var a;
             b=9;
            console.log(a);  //undefined
            console.log(b);  //9
            a='123';
        }
         a=18;
         f1();
         */
    </script>
</head>

<body>

</body>

</html>